STABILITY, RHEOLOGY AND THE EFFECT OF VARIOUS ENVIRONMENTAL FACTORS ON THE FORMATION OF FOOD PROTEIN-STABILIZED NANOGELS FROM NANOEMULSIONS - Aakash Patel M.Sc. Thesis

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Masters

Abstract

Nanoemulsions (NEs) typically have average droplet size less than 200 nm which provides them many characteristics advantages over conventional emulsions. NEs have higher stability and form nanogels at a lower oil volume fraction than conventional emulsions. However, so far, all nanogels were formed with small molecular weight surfactants, such as sodium dodecyl sulfate (SDS). The primary objective of this thesis was to achieve gelation in oil-in-water (O/W) NEs stabilized with sodium caseinate (SC) and whey protein isolate (WPI) and to investigate their gelation behavior as influenced by long-term storage, addition of salt, change in pH and heat treatment.
SC and WPI-stabilized NEs with different protein concentration (2 - 5 wt%) and different oil concentrations (30 & 40 wt%) were prepared by multiple cycles of high-pressure homogenization. Only SC-stabilized NEs (SC NEs) formed strong elastic gel at 5% protein and 40% oil. All the other SC NEs at 40% oil showed weak gel behavior while no gelation behavior was observed for any of the WPI-stabilized NEs (WPI NEs). The droplet interaction potential calculation indicated that the longer steric layer of SC, in combination with the strong electrostatic barrier provided a thicker interfacial layer in SC-stabilized droplets compared to WPI. This increase in interfacial layer thickness led to an increase in effective volume fraction (eff) towards ~0.7 for SC NEs, which caused a close packing of droplets leading to repulsive gelation in SC NEs. All the NEs showed stability in droplet size, while their viscosity and gel strength remained unchanged over a period of 3 months.
Next, selected NEs, which showed weak to no gelation behavior (2 & 4% SC or WPI and 30 & 40% oil) were further investigated by the addition of salt, change in pH and heat-treatment to induce attractive gelation. WPI NEs showed gelation upon addition of salt due to a screening of charge which led to attractive interaction between the droplets. However, SC NEs did not show any gelation due to their longer hydrophilic tail which provided a strong steric barrier against salt-induced attractive interaction. NEs with both proteins showed gelation at pH near the isoelectric point which was contributed to the charge neutralization. Heat-treatment did not cause any gelation in any of the NEs, despite WPI being a heat-labile protein. This was attributed to the lack of protein in the continuous phase to cause any heat-induced gelation.
Overall, the study showed that it is possible to develop repulsive gels from SC NEs due to its high interfacial layer thickness and smaller droplet size even at an oil volume fraction of 0.4, which was much lower compared to the oil volume fraction of greater than 0.64 for conventional emulsion. Addition of salt or change in pH towards the protein’s isoelectric point may lead to attractive gelation in certain NEs, provided the interfacial steric barrier prevent close interaction of the droplets. However, heat-treatment could not induce gelation in the NEs due to a lack of protein in the continuous phase.